Multilayer ceramic substrates have been used in packages containing semiconductor devices and the like, interposers, antenna switch modules in which a front-end circuit for communication devices is incorporated, amplifier modules, filter modules, antennas, etc.
In the multilayer ceramic substrate, a plurality of ceramic layers are stacked up, and a wiring layer which is realized by an electrode pattern formed by screen printing or a reactance function layer which forms inductance and capacitance is provided between respective layers. Electrode patterns provided in different gaps between the ceramic layers are appropriately connected to one another by via wires penetrating through the ceramic layers. The connection by the via wires can be realized across three or more layers in some cases by aligning the via wires in the thickness direction of the multilayer ceramic substrate. The via wire can be used as a thermal via in some cases such that the thermal via transfers heat produced in a semiconductor of an amplifier or the like incorporated in the multilayer ceramic substrate to the circuit board side.
In such a multilayer ceramic substrate, it has been required to form a plurality of via wires with high density within a limited area for the sake of size reduction of various electronic parts. Also, in recent years, while the wire width has been narrowing due to increase in density of the wire pattern and the like in circuit designing, it has been required to reduce the distance between a via wire and a line connected with the via wire, such as coplanar lines and ground coplanar lines, and an adjoining ground electrode.
The multilayer ceramic substrate is manufactured by stacking up ceramic green sheets in which electrodes, a pattern of wires and a via wire are formed. The via wire is formed by forming a via hole in a ceramic green sheet and filling a conductor paste into the via hole. In this process, to secure assured injection of the conductor paste into the via hole and assured connection of the via wires, the conductor paste is provided in a region which is slightly larger than the via hole and filled into the via hole so as to be higher than the opening of the via hole. Part of the conductor projecting out of the via hole is called “pad”.
Patent Document 1 discloses using a via wire which has no pad for the sake of high density formation of via wires. Patent Document 2 discloses stacking up two or more ceramic green sheets and thereafter forming a via hole through the resultant multilayer structure, and filling a conductor paste into the via hole, thereby forming a via wire stacked up without forming a pad between the layers.